The present invention relates generally to a position measuring apparatus, and, more particularly, to a position measuring apparatus having a subdivision circuit for subdividing sinusoidal scanning signals.
Incremental measuring systems, as is well-known, deliver impulses whose number is a measure of the linear displacement or the angle of rotation of an object being measured. These impulses are summed as path elements in a counter and are numerically displayed as a measurement value or fed to a processing machine for further processing.
The graduation of a scale is scanned by a scanning unit, so there are generated two analog scanning signals phase-shifted relative to one another by 90.degree.. By triggering these sinusoidal scanning signals there are obtained two phase-shifted impulse sequences, from the evaluation of which there is made possible a subdivision of a graduation period or of a scanning signal period with the factor 4.
For many applications a higher subdivision factor, i.e., a higher resolving capacity, is required.
If the two analog scanning signals have a sufficiently good sinusoidal form, they can be subdivided a relatively large number of times. This subdividing is also called interpolation.
A circuit for interpolation is described on pages 63-70 of the dissertation of Dipl.-Phys. Gerd Ulbers: "A Laser Interferometer for Photoelectric Movement Telemetry in Both Lateral Coordinates" of 1981. This circuit is based on the use of two directly dividing analog-to-digital converters. Each of these analog-to-digital converters makes available the ratio of reference voltage to input voltage as a digital result at its output. The division thus performed corresponds to the tangent formation of the analog scanning signals if the larger signal amplitude of the two analog scanning signals lies on one input and the smaller signal amplitude of the other scanning signal lies on the other input of the analog-to-digital converter. In order to achieve this, two parallel analog-to-digital converters are provided which are alternately driven in dependence on the signal amplitudes of the two analog scanning signals. Since the analog-to-digital converters cannot process a negative reference signal, the amount is formed in a network in each case from the two analog scanning signals.
By the analog-to-digital converters there is generated an 8-bit data word which presents an address for a PROM, under which the corresponding arc-tangent value is given out. This value defines the position within a quarter graduation period or signal period of the scanning signals.
What is disadvantageous in this known circuit is the relatively high component costs and the alternating activation of two analog-to-digital converters.